The invention relates to a catalyst with improved catalytic properties, particularly a catalyst suitable for the preparation of epoxides.
Methods have been described for lowering the total concentration of soluble species in the bulk of a catalyst carrier. These methods generally involve a process by which the carrier is manufactured in such a way so as to lower the concentration of those species throughout the bulk of the carrier. These approaches limit the formulation of carriers, often times with undesirable consequences such as high carrier density.
U.S. Pat. No. 4,797,270 discloses water washing to reduce the sodium content of an alumina powder. The pH of the wash water may need to be adjusted for extraction of other metals and Japanese patent JP56164013 discloses the use of a low pH (acid) to extract uranium and thorium from a calcined xcex1-alumina raw material.
U.S. Pat. Nos. 4,361,504 and 4,366,092 suggest that ethylene oxide catalyst be water washed after the deposition of silver or silver/gold on the carrier. EP-211521 discloses washing of a catalyst with hot water to remove basic materials left on the catalyst from a silver impregnation process or the physical deposition of alkali metals. U.S. Pat. No. 4,367,167 discloses a process for a supported catalyst wherein an impregnated support is immersed in an inert water immiscible organic solvent containing a dissolved aliphatic amine. U.S. Pat. No. 4,810,689 discloses depositing a silver compound, decomposing the silver compound to silver in the presence of an alkali metal compound, removing organic deposits by washing and introducing fresh alkali metal by impregnation during or after the washing stage. U.S. Pat. Nos. 4,186,106 and 4,125,480 disclose washing with an inert liquid after deposition of the catalytic metal and before deposition of a promoter material.
The prior art remains concerned with the total amount of impurities; i.e., impurities throughout the bulk. Unfortunately, the impurity removal techniques taught typically attack the carrier itself. It has surprisingly been found that controlling the solubilization rate of certain species found on a carrier surface results in a catalyst with improved catalytic properties.
According to the invention, there is provided a catalyst carrier comprising a material having a sodium solubilization rate no greater than 5 ppmw/5 minutes.
Another embodiment of the invention provides a catalyst comprising a carrier having a sodium solubilization rate no greater than 5 ppmw/5 minutes; and one or more catalytically reactive metals deposited on said carrier. A further embodiment of the invention provides a catalyst suitable for the vapor phase production of epoxides comprising a carrier having a sodium solubilization rate no greater than 5 ppmw/5 minutes; and one or more catalytically reactive metals deposited on said carrier.
A further embodiment of the invention provides a catalyst suitable for the vapor phase production of oxiranes from olefin and oxygen comprising a carrier having a sodium solubilization rate no greater than 5 ppmw/5 minutes; and catalytically reactive silver deposited on said carrier.
It has been found that carriers which have a controlled solubilization rate, in particular controlled sodium and/or soluble silicate solubilization rates, provide catalysts with improved catalytic properties, such as activity, selectivity and activity and/or selectivity performance over time. Controlling the solubilization rate is believed to work to improve the properties of most catalysts, no matter how impure the bulk carrier material. Further, controlling the solubilization rate will work for organic or inorganic carriers.
The typical carrier of the invention has a sodium solubilization rate in boiling water which is controlled to be no greater than 5 ppmw/5 minutes. As used herein, boiling water is deemed to have a temperature of 100xc2x0 C. xe2x80x9cSolubilization ratexe2x80x9d as used herein refers to the measurable solubilization rate of the sodium in a solution after the carrier is placed in the solution for a specified time and at a ratio of boiling solution to carrier of 3:1. Thus, a solubilization rate in boiling water of 5 ppmw sodium/5 minutes is the amount of sodium measured in the water after the carrier has been in the boiling water for five minutes.
Carriers are commonly inorganic materials such as, for example, alumina-, silica-, or titania-based compounds, or combinations thereof, such as alumina-silica carriers. Carriers may also be made from carbon-based materials such as, for example, charcoal, activated carbon, or fullerenes. Ionizable species typically present on the inorganic type carriers include sodium, potassium, aluminates, soluble silicate, calcium, magnesium, aluminosilicate, cesium, lithium, and combinations thereof. Of particular concern are the ionizable anionic species present on the surface, particularly ionizable silicates. The solubilization rate of silicates may be measured by inductively coupled plasma (ICP) techniques and the amount of silicon species on a surface may be measured by x-ray photoelectron spectroscopy (XPS); however, since sodium is soluble in the same solutions that silicates are soluble in, the solubilization rate of sodium becomes a simpler check of the ionic species removal and it has been chosen as the indicator to define the present invention. Another measurement technique is to measure the electrical conductivity of the treatment solution.
Control of the solubilization rate may be obtained by a multiple of means. The raw materials for the carrier can be tightly controlled, for example. Or the surface of the carrier may be treated. As used herein, the xe2x80x9csurfacexe2x80x9d of the carrier is that area of the carrier which may be measured by BET analysis. Specifically, the surface of the carrier is the site at which reaction takes place. Lowering the concentration of ionizable species on the surface of the carrier has been found to be an effective and cost efficient means of achieving the desired sodium solubilization rate. An xe2x80x9cionizablexe2x80x9d species is a species which is capable of being rendered ionic, where the term xe2x80x9cionicxe2x80x9d or xe2x80x9cionxe2x80x9d refers to an electrically charged chemical moiety.
Lowering the surface solubilization rate of ionizable species may be accomplished by any means (i) which is effective in rendering the ionizable species ionic and removing the species, or (ii) which renders the ionizable species insoluble, or (iii) which renders the ionizable species immobile; however, use of aggressive medias is discouraged as these medias tend to dissolve the carrier, extract too much material from the bulk, and generate acidic or basic sites in the pores. Acids, which are considered aggressive media, will remove the cations on a carrier but are fairly ineffectual in removing the undesirable anions, such as silicates. Effective means of lowering concentration include washing the carrier; ion exchange; volatilizing, precipitating, or sequestering the impurities; causing a reaction to make the ionizable species on the surface insoluble; and combinations thereof. The bulk carrier may be treated, or the raw materials used to form the carrier may be treated before the carrier is manufactured. Even greater improvements in solubilization rate control are seen when both the carrier raw materials and the finished carrier are treated.
To make a catalyst from the carrier, the carrier is typically impregnated with metal compound(s), complex(es) and/or salt(s) dissolved in a suitable solvent sufficient to deposit or impregnate a catalytically effective amount of metal on the carrier. As used herein, xe2x80x9ccatalytically effective amountxe2x80x9d means an amount of metal that provides a measurable catalytic effect. For example, a catalytically effective amount of metal when referring to an olefin epoxidation catalyst is that amount of metal which provides a measurable conversion of olefin and oxygen to alkylene oxide. In addition, one or more promoters may also be deposited on the carrier either prior to, coincidentally with, or subsequent to the deposition of the catalytically reactive metal. The term xe2x80x9cpromoterxe2x80x9d as used herein refers to a component which works effectively to provide an improvement in one or more of the catalytic properties of the catalyst when compared to a catalyst not containing such component.
Further improvement in the catalyst properties are seen when the metal deposition is effected by contacting the carrier with an impregnation solution whose hydrogen ion activity has been lowered. xe2x80x9cHydrogen ion activityxe2x80x9d as used herein is the hydrogen ion activity as measured by the potential of a hydrogen ion selective electrode. As used herein, a solution with xe2x80x9cloweredxe2x80x9d hydrogen ion activity refers to a solution whose hydrogen activity has been altered by the addition of a base, such that the hydrogen ion activity of the altered solution is lowered compared to the hydrogen ion activity of the same solution in an unaltered state. The base selected to alter the solution may be chosen from any base or compound with a pKb lower than the original impregnation solution. It is particularly desirable to chose a base which does not alter the formulation of the impregnation solution; i.e., which does not alter the desired metals concentration in the impregnation solution and deposited on the carrier. Organic bases will not alter the impregnation solution metals concentrations, examples of which are tetraalkylammonium hydroxides and 1,8-bis-(dimethylamino)-naphthalene. If changing the metals concentration of the impregnation solution is not a concern, metal hydroxides may be used.
When the impregnation solution is at least partially aqueous, an indication of the change in the hydrogen activity may be measured with a pH meter, with the understanding that the measurement obtained is not pH by a true, aqueous definition. xe2x80x98xe2x80x9cMeasured pHxe2x80x9dxe2x80x99 as used herein shall mean such a non-aqueous system pH measurement using a standard pH probe. Even small changes in the xe2x80x9cmeasured pHxe2x80x9d from the initial impregnation solution to that with added base are effective and improvements in catalytic properties continue as the xe2x80x9cmeasured pHxe2x80x9d change increases with base addition. High base additions do not seem to adversely affect catalyst performance; however, high additions of hydroxides have been seen to cause sludging of the impregnation solution, creating manufacturing difficulties. When the base addition is too low, the hydrogen ion activity will not be affected. The hydrogen ion activity lowering procedure is also quite effective when used by itself; i.e., when no ionizable species concentrations are lowered prior to impregnation.
The impregnated carrier, also known as a catalyst precursor, is dried in the presence of an atmosphere which also reduces the catalytic metal. Drying methods known in the art include steam drying, drying in an atmosphere with a controlled oxygen concentration, drying in a reducing atmosphere, air drying, and staged drying using a suitable ramped or staged temperature curve.
By way of example, the invention will be described in more detail for a catalyst suitable for the vapor phase production of epoxides, also known as an epoxidation catalyst.
An epoxidation catalyst typically comprises an inorganic carrier, such as for example, and alumina-based carrier such as xcex1-alumina, with one or more catalytically reactive metals deposited on the carrier. The carrier typically contains certain ionizable species, for example an xcex1-alumina carrier, typically contains species including sodium, potassium, aluminates, soluble silicates, calcium, magnesium, aluminosilicates, and combinations thereof. It has been found that silicates, and certain other anions, are particularly undesirable ionizable species in an epoxidation catalyst. As already described, the solubilization rate of silicons/silicates may be measured by ICP and by XPS; however, since sodium is soluble in the same solutions that silicates are soluble in, the solubilization rate of sodium becomes a simpler check of the ionic species removal. Another measurement technique is to measure the electrical conductivity of the treatment solution.
According to the invention, the sodium solubilization rate of the carrier is controlled. The solubilization rate may be controlled by lowering the concentration of ionizable species on the surface. Ionizable species concentration may be lowered by means which render the ionizable species ionic and thereafter removing the ionic species, or by rendering those ionizable species insoluble, or rendering the ionizable species immobile. For example, the carrier, or the raw materials of the carrier, may be subjected to washing; ion exchange; volatilizing, precipitating, or sequestering the impurities; causing a reaction to make the ionizable species on the surface insoluble; and combinations thereof. When washing is used, the sodium solubilization rate in 3:1 w/w boiling water is preferably controlled to less than 5 ppmw Na/5 minutes.
The carrier having the controlled solubilization rate is impregnated with metal ions or compounds), complex(es) and/or salt(s) dissolved in a suitable solvent sufficient to cause the desired deposition on the carrier. When silver is the deposition material, a typical deposition is from about 1 to about 40 percent by weight, preferably from about 1 to about 30 percent by weight silver, basis the weight of the total catalyst. The impregnated carrier is subsequently separated from the solution and the deposited metal(s) compound is reduced to metallic silver.
One or more promoters may be deposited either prior to, coincidentally with, or subsequent to the deposition of the metal. Promoters for epoxidation catalysts are typically selected from sulfur, phosphorus, boron, fluorine, Group IA through Group VIII metals, rare earth metals, and combinations thereof. The promoter material is typically compound(s) and/or salt(s) of the promoter dissolved in a suitable solvent.
For olefin epoxidation catalysts, Group IA metals are typically selected from potassium, rubidium, cesium, lithium, sodium, and combinations thereof; with potassium and/or cesium and/or rubidium being preferred. Even more preferred is a combination of cesium plus at least one additional Group IA metal, such as cesium plus potassium, cesium plus rubidium, or cesium plus lithium. Group IIA metals are typically selected from magnesium, calcium, strontium, barium, and combinations thereof, Group VIII transition metals are typically selected from cobalt, iron, nickel, ruthenium, rhodium, palladium, and combinations thereof; and rare earth metals are typically selected from lanthanum, cerium, neodymium, samarium, gadolinium, dysprosium, erbium, ytterbium, and mixtures thereof. Non-limiting examples of other promoters include perrhenate, sulfate, molybdate, tungstate, chromate, phosphate, borate, sulfate anion, fluoride anoin, oxyanions of Group IIIB to VIB, oxyanions of an element selected from Groups III through VIIB, alkali(ne) metal salts with anions of halides, and oxyanions selected from Groups IIIA to VIIA and IIIB through VIIB. The amount of Group IA metal promoter is typically in the range of from about 10 ppm to about 1500 ppm, expressed as the metal, by weight of the total catalyst, and the Group VIIb metal is less than about 3600 ppm, expressed as the metal, by weight of the total catalyst.
For further improvement in catalytic properties, the hydrogen ion activity of the impregnation solution is optionally lowered, such as by the addition of a base. The typical impregnation solution for an epoxidation catalyst begins quite basic, so a strong base is used to further lower the hydrogen ion activity. Examples of strong bases include alkyl ammonium hydroxide such as tetraethylammonium hydroxide, and metal hydroxide such as lithium hydroxide and cesium hydroxide. In order to maintain the desired impregnation solution formulation and metal loading, an organic base such as tetraethylammonium hydroxide is preferred. Base additions in these systems typically result in a xe2x80x9cmeasured pHxe2x80x9d change ranging up to about 3 pH units, realizing that the xe2x80x9cmeasured pHxe2x80x9d is not a true pH since the impregnation system is not aqueous.
The carrier employed in these catalysts in its broadest aspects can be any of the large number of conventional, porous refractory catalyst carriers or carrier materials which are considered relatively inert. Such conventional materials are known to those skilled in the art and may be of natural or synthetic origin. Carriers for epoxidation catalysts are preferably of a macroporous structure and have a surface area below about 10 m2/g and preferably below about 3 m2/g. Examples of carriers for different catalysts are the aluminum oxides (including the materials sold under the trade name xe2x80x9cAlundumxe2x80x9d), charcoal, pumice, magnesia, zirconia, kieselguhr, fuller""s earth, silicon carbide, porous agglomerates comprising silica and/or silicon carbide, silica, magnesia, selected clays, artificial and natural zeolites, alkaline earth carbonates, and ceramics. Refractory carriers especially useful in the preparation of olefin epoxidation catalysts comprise the aluminous materials, in particular those comprising xcex1-alumina. In the case of xcex1-alumina-containing carriers, preference is given to those having a specific surface area as measured by the B.E.T. method of from about 0.03 m2/g to about 10 m2/g, preferably from about 0.05 m2/g to about 5 m2/g, more preferably from about 0.1 m2/g to about 3 m2/g, and a water pore volume as measured by conventional water absorption techniques of from about 0.1 to about 0.75 cc/g by volume. The B.E.T. method for determining specific surface area is described in detail in Brunauer, S., Emmett, P. Y. and Teller, E., J. Am. Chem. Soc., 60, 309-16 (1938).
Certain types of xcex1-alumina containing carriers are particularly preferred. These xcex1-alumina carriers have relatively uniform pore diameters and are more fully characterized by having B.E.T. specific surface areas of from about 0.1 m2/g to about 3 m2/g, preferably from about 0.1 m2/g to about 2 m2/g, and water pore volumes of from about 0.10 cc/g to about 0.55 cc/g. Manufacturers of such carriers include Norton Chemical Process Products Corporation and United Catalysts, Inc. (UCI).
The resulting epoxidation catalysts just described are used for the vapor phase production of epoxides. A typical epoxidation process involves loading catalysts into a reactor. The feedstock to be converted, typically a mixture of ethylene, oxygen, carbon dioxide, nitrogen and ethyl chloride, is passed over the catalyst bed at pressure and temperature. The catalyst converts the feedstock to an outlet stream product which contains ethylene oxide. Nitrogen oxides (NOx) may also be added to the feedstock to boost catalyst conversion performance.
Having generally described the invention, a further understanding may be obtained by reference to the following examples, which are provided for purposes of illustration only and are not intended to be limiting unless otherwise specified.